Poster presented at 13th Pisa Meeting on Advanced Detectors - La Biodola, Isola d'Elba (Italy) - May 25 - 30, 2015

S8 - Detector Techniques for Cosmology, Astroparticle and General Physics

A scintillating bolometer array for double beta decay studies:

the LUCIFER experiment

Luca Gironi on behalf of the LUCIFER collaboration

2νDBD of 100Mo

The Low‐background Underground Cryogenics Installation For Elusive Rates (LUCIFER)

ZnMoO4

Reconstruction of the

background spectra. The best fit

line (solid blue) is shown. The

components are 2νDBD (gray

region), 65Zn (light blue),

internal and external 210Pb

(yellow), internal and external 40K (green), 208Tl (pink), and flat

α background (orange).

Neutrinoless double beta decay

Importance of the process:

- It can happen only if ν is a Majorana particle

- From the decay half-life we can measure the effective

Majorana mass mββ:

The LUCIFER setup will consist of an array of 30 individual module detectors, arranged in a tower-like structure installed underground in the Laboratori Nazionali del Gran Sasso. Each module will be

composed by a ~0.5 kg enriched (95%) ZnSe crystal equipped with a Ge-crystal light detector operated as bolometers at ~10 mK. The goal of Lucifer is to reach sensitivity of few 1025 y and to be a

demonstrator for a background free experiment.

ZnSe crystal

Light Detectors

The large statistics collected during the operation of a ZnMoO4 array, for a total exposure of 1.3 kg day of 100Mo, allowed the first bolometric observation of the 2νDBD of 100Mo. The analysis of coincidences

between the crystals allowed the assignment of constraints to the intensity of the different background sources, resulting in a reconstruction of the measured spectrum down to an energy of ∼300 keV.

Crystal mass (g) Anticoincidence Coincidence

247 509 ± 26 4.4 ± 0.2

235 472 ± 24 5.4 ± 0.3

329 661 ± 34 6.2 ± 0.3

Number of events from the 2νDBD of 100Mo in the anticoincidence

spectrum and in the coincidence spectrum for each crystal.

Experimental approach: the scintillating bolometers

Nuclear decay:

G0ν = phase space factor

M0ν = nuclear matrix element

Sensitivity (S): the process half-life corresponding to the maximum signal that could be observed (over the background fluctuations) at a given

statistical Confidence Level (C.L.).

Looking for a monochromatic peak

at the Q-value of the decay.

Successful R&D pursued within LUCIFER with Mo-based compound

before choosing ZnSe.

Our light detector is a Ge thin crystal operated as bolometer. This device is calibrated through an ionizing 55Fe source.

Ø=44.5 mm, h=0.175 mm

They demonstrate the very

appealing possibility to

discriminate particle

through pulse shape analysis

on the heat channel.

Smeared α source

β/γ events

Uei = PMNS matrix elements

mi = mass eigenvalues

In order to achieve a high sensitivity we need: excellent energy resolution, large source mass, very low background.

The leading idea: to combine the two available

information (heat and scintillation light) to

distinguish the nature of the interacting particles,

exploiting the different scintillation yield of β/γ, α

and neutrons.

Scintillating bolometer: a bolometer coupled to a light detector. The first consists of a scintillating absorber thermally linked to a phonon

sensor while the latter can be any device able to measure the emitted photons.

Example of scatter plot obtained with scintillating

crystals.

Isotope Q-value i.a.

100Mo 3034 keV 9.6 %

Isotope Q-value i.a.

82Se 2996 keV 8.7 %

n = confidence level

Nββ = isotope number

T = live time

M = detector mass

B = background

∆ = energy resolution

Lig

ht

shap

e p

ar.

[a

.u.]

Energy [keVee]

β/γ events

smeared α source

82Se Qββ

DP(Qββ) = 11

σ = 96 eV

55Fe

ieiββ

mUm2

200

2

2

0

21

1

MG

m

m

Te

/

eZAZA 22,,

ysyststatT181002

2110660370157 )(.)(..)Mo(

/

n

TNS

2ln

BM

TNS

2ln

B

822

.ln

0B

TNS

)C.L. %( . 6882n

‘Zero’ background experiments

Experiments with background

)C.L. %( 68 BTMn

Performances as a function of the

sensor polarization current.

Det

ecte

d l

igh

t [k

eV]

Energy [keVee]

82Se Qββ

ZnSe peculiarity: α

particles scintillate

more than β/γ’s:

Q.F.≈4

431g crystal

)()(

)()(

)(

/

/

EE

EE

EDP22

Discrimination Potential

The light emitted and the shape of the same light pulses are reported as a function of

the energy released in the ZnSe bolometer. A 𝛾-source was used to produce events (blue)

up to 2615 keV (208Tl) while a smeared 𝛼 source was placed under the crystal to provide

a continuum of 𝛼’s extending to lower energies (red). FWHM(2615keV)=13.4±1.3 keV

FWHM(2615keV)=6.3±0.5 keV

DP(Qββ) = 20

without light detection

2νDBD 0νDBD

Energy [keV]

Co

un

ts

FW

HM

(eV

) R

ise

Tim

e (m

s)

Ibol (pA)

Main advantages of the bolometric technique:

- Possibility to choose different 0νDBD candidate

- Excellent energy resolution (~0.1%)

- Source = Detector

Background reduction

without a double readout.